Satellite positioning and the prediction of satellite positions are used extensively for many applications, for example vehicle navigation systems and portable GPS devices. For example, in order to compute a position, GPS receivers on GPS devices require the positions of the GPS satellites at the time ranging signals (i.e. the time tag of a signal at the time it is transmitted from the satellite antenna). This satellite orbit information is provided by the satellites on a radio frequency (RF) data link in the form of a satellite position model. The model uses a set of orbital elements, known as the ‘Ephemeris’, which is valid for a limited time period, typically 4 hours, but may be used for as long as 6 hours. The GPS satellites broadcast the Ephemeris data on an RF data link, and a GPS receiver continuously monitors and demodulates this data stream to obtain the updated Ephemeris. Ephemeris transmitted from a satellite is known as “Broadcast Ephemeris”.
The Ephemeris data is a mathematical orbit arc model that allows a GPS device to evaluate a set of equations, and obtain the satellite position at any time during the four to six hour model fit period. Although the model allows for the evaluation of the satellite position beyond the 4 to 6 hours of validity, the accuracy typically degrades to the level of about a kilometer within a day. For a more detailed description of GPS and the ephemeris model, see “Global Positioning System: Theory and Applications” edited by Parkinson and Spilker, Vol. 1, chapters 2 (signal structure), 4 (ephemeris model), and 9 (navigation solutions).
In the case of GPS, it typically takes about 18 to 30 seconds to demodulate a Broadcast Ephemeris received from a particular satellite in good observation conditions and if the Ephemeris is demodulated on a first pass. Jamming and/or attenuated RF environments, such as urban environments or indoor locations, make demodulation spotty or difficult. This impacts the user experience by incurring a Time-To-First-Fix (TTFF) of 45 seconds or longer (there is a possibility of no fix at all), and battery life of the GPS device may be shortened. The TTFF is the time required for a GPS device to acquire satellite signals and navigation data, and calculate a position solution. For a cold start of a GPS device the TTFF can be over 15 minutes. In some cases GPS signals are too weak to faithfully demodulate navigation data, yet they are strong enough to be tracked using the current generation of receivers. If these receivers are provided with an alternate source of Ephemeris, instead of relying on demodulation of the Broadcast Ephemeris, performance of the GPS device can be improved, and TTFF can be reduced to a few seconds, even under attenuated conditions, and battery life will be extended. The GPS device can also calculate a fix by using no Z count techniques, thereby increasing the use of the navigation device.
The technique of providing an alternative source of Ephemeris, besides the Broadcast Ephemeris, is generally referred to as Assisted-GPS (AGPS). There are several types of AGPS available, including Real-Time Assistance techniques and Synthetic Assistance (also known as Predictive or Extended Ephemeris) techniques. Real-Time Assistance techniques deliver actual Broadcast Ephemeris that is pre-collected from a network of fixed GPS reference stations that relay to a central data center all the current Broadcast Ephemeris received from every satellite in view at each reference station. This reference data (or Assistance) is transformed by an AGPS server into a format that can be delivered to the GPS device over a communications network connection.
Rather than collect and relay real Broadcast Ephemeris data in real-time, Synthetic Assistance techniques use an AGPS server to predict or synthesize satellite position data (or Assistance) days or weeks into the future and deliver this non-real time, Synthetic Assistance data to GPS devices over a communications network connection or a direct connection with a host, such as a personal computer. An important limitation of the AGPS techniques discussed above is that they require some form of connection, network or direct, to a server, to download Assistance data to the GPS device.
Some GPS devices lack external connection capability, network or otherwise, and in some cases, a connection-capable device may be unable to establish a connection for a long period of time to a server. Under these conditions, GPS devices cannot use the AGPS techniques mentioned above and performance is impacted. Additionally, devices using Real-Time Assistance techniques consume network resources when downloading the Assistance. If the Broadcast Ephemeris validity period is extendable for a longer period of time, beyond the normal 4 to 6 hour window, network overhead could be reduced and TTFF performance improved for most GPS device use cases. The GPS industry has shown some efforts to extend the usability period of Broadcast Ephemeris by attempting to directly predict the future values of the various Keplerian terms of the Broadcast Ephemeris model. However, operating within the Keplerian model severely limits the ability of a GPS device to reliably predict these values beyond a day or so.
Another technique to provide AGPS Assistance is disclosed in U.S. patent application Ser. No. 11/740,206 for a “Distributed Orbit Modeling and Propagation Method for a Predicted and Real-Time Assisted GPS System”. In this system, the GPS device, i.e. the client, predicts the Synthetic Assistance data itself, but does so after periodic receipt of enabling data (called “Seed Data”) from a remote server.
Precise orbit modeling is known in the art. Predicting, i.e. propagating the position and velocities of satellites to future points in time is a function of analyzing current and/or past real Orbit State Vector samples, combined with a choice of specific force models that affect orbit trajectories. Typical orbit modeling considerations include the effects of a wide variety of force models, including those of the sun, moon and the earth. The software implementation of such orbit models is typically in the form of a Integrator which, using current and/or prior real Orbit State Vector samples, can propagate these to future points in time. Due to the CPU-intensive nature of evaluating the force models and tying the force models into a common reference frame, this code is made for operation on server-class computers.
The prior art techniques do not provide a GPS device the ability to propagate orbital information with the required accuracy for position computation. The prior art associated with providing Ephemeris data to the GPS device has been constrained to the Keplerian mathematical model used in the Broadcast Ephemeris data received from the satellite. To provide Assistance, the prior art requires a server to generate Assistance information and that the GPS device have some form of network connection for receiving the Assistance. The prior art does not enable a GPS device to use Broadcast Ephemeris as direct input to synthesize its own assistance data.